I am trying to make an animation of a double planet, can't seem to get it right.
My solution is to use an empty (Empty1) as the Moon/Planet barycentre which follows a large path representing the star orbit. The moon and Planet each have a path that follows Empty1 around the Star.





Here you can see a Constraint example:



These are the loops indicating days and a year:



Of course the large orbit path should also be eccentric but I doubt that the difference in mass here would be noticeable ;-)